Segment information - Customer Revenue by Classes of Similar Products or Services (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue from External Customer [Line Items]
Revenue	$ 721.7	$ 683.3	$ 748.9	$ 666.8	$ 789.9	$ 739.2	$ 764.8	$ 721.2	$ 2,820.7	$ 3,015.1	$ 3,356.4
Services
Revenue from External Customer [Line Items]
Revenue									2,406.3	2,605.6	2,785.7
Technology
Revenue from External Customer [Line Items]
Revenue									414.4	409.5	570.7
Cloud & infrastructure services | Services
Revenue from External Customer [Line Items]
Revenue									1,352.9	1,513.1	1,704.9
Application services | Services
Revenue from External Customer [Line Items]
Revenue									859.0	868.9	819.8
BPO services | Services
Revenue from External Customer [Line Items]
Revenue									$ 194.4	$ 223.6	$ 261.0